1. Field of the Invention
The present invention relates to a switching power source for generating a direct-current voltage.
2. Description of the Related Art
With an increase in demand for various power-saving electronic devices in recent years, more power saving is requested of power sources for the electronic devices. As an example of the power source for the electronic device, there is used a power source based on a switching system (hereinafter, switching power source) that outputs a target voltage by driving a switching element such as a field effect transistor (FET) with a predetermined drive frequency.
Such switching power sources include a power source that improves efficiency by reducing the number of switching operations of the switching element during a power saving operation (during light load running). There have also been yearly changes in power saving standard, and improvement of efficiency is requested by saving power during the light load running other than during normal running.
Most losses of the switching power source during the light load running are caused by a switching operation. Efforts have therefore been made to reduce the losses caused by the switching operation. For example, energy of one switching operation is increased by prolonging an ON-period of the switching element, and the number of switching times per unit time is reduced by prolonging a pause period.
However, the longer pause period reduces a switching frequency. The sound generated due to the lower switching frequency enters an audible range, possibly reaching human ears. The sound generated due to the lower switching frequency becomes a sound including a harmonic, which is disagreeable to the ear.
The reason why the lower switching frequency generates the sound including the harmonic is described below. When the switching frequency drops to several kilohertz or less, the pause period of the switching element becomes longer. Thus, a drive current waveform of a transformer becomes a delta-function waveform illustrated in FIG. 10.
Frequency analysis of such a drive current waveform of the transformer reveals that the waveform has a frequency component containing a harmonic of 100 kilohertz or more with the switching frequency set as a fundamental wave. FIG. 11 illustrates frequency characteristics of the drive current waveform illustrated in FIG. 10. As illustrated in FIG. 11, the waveform becomes a current waveform having energy driven with a multiple higher frequency (harmonic) of the switching frequency.
The transformer of the switching power source also performs a switching operation to be driven with a predetermined resonance frequency. The resonance frequency generated by mechanical vibrations of the transformer has, though dependent on a core shape of the transformer, a peak approximately at several kilohertz to several tens of kilohertz.
FIG. 12 illustrates a resonance frequency generated by mechanical vibrations of the transformer. FIG. 5B illustrates a result of measuring, by using a microphone, a sound generated when the transformer having characteristics illustrated in FIG. 12 is driven based on the drive current waveform illustrated in FIG. 11 and analyzing frequency characteristics.
As illustrated in FIG. 5B, a beat sound generated from the transformer characteristically contains a harmonic where the envelope exhibits resonance characteristics of the transformer with an intermittent switching frequency set as a basic wave. FIG. 5A illustrates a waveform of a drive signal input to the transformer.
As illustrated in FIG. 5B, when the switching frequency and the mechanical resonance frequency of the transformer overlap each other to lower the switching frequency, a sound that enters the audible range as a beat sound from the transformer is generated.
As one of methods for reducing such a beat sound from the transformer, a method for reducing the beat sound by suppressing a magnetic field change rate of the transformer is well-known. Conventionally, to suppress the magnetic field change rate of the transformer, there has been employed a method for using a core material having a large sectional area for the transformer or reducing a current per one transformer operation by shortening the ON-period of the switching element.
As a method for reducing the beat sound of the transformer by ingeniously setting the drive current waveform of the transformer, there is a method for gradually changing a duty ratio during voltage rising or falling at both ends of a capacitor at the time of activation by installing a soft start circuit in a switching power source apparatus. Gradually increasing or decreasing the amplitude of the drive current waveform of the transformer enables reduction of a magnetic flux change of the transformer, and hence generation of beat sounds can be reduced. Such a conventional method is discussed in, for example, Japanese Patent No. 3567355 or Japanese Patent No. 3665984.
However, the use of the core material large in sectional area for the transformer causes an increase in size of the transformer, resulting in a difficulty of downsizing the switching power source or the apparatus that includes the switching power source. The method for shortening the ON-period of the switching element can reduce the beat sound of the transformer because the shorter ON-period reduces the magnetic field change. However, the number of switching times per unit time increases, and hence switching losses increase.
In the case of the method for gradually increasing or decreasing the size of the drive current waveform of the transformer, when power consumption is reduced more, application of the method is difficult if energy supplied to a load of a secondary side is small. It is because during the light load running, it is difficult to gradually increase or decrease the amplitude of the current waveform by the soft start circuit.
In the conventional method, switching must be performed more times by reducing energy supplied for one switching operation, or the capacity of a capacitor of the secondary side must be increased several times without changing the energy supplied for one switching operation. The former method increases switching losses, greatly lowering efficiency. The latter method increases costs.
In other words, in the switching power source, reduction of switching losses by decreasing the number of switching times, is required. However, this case has a contradiction, namely, a larger sound generated from the transformer because energy per wave applied to the transformer is increased.